78 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
My own study has shown that the typical ashbed-diabases of Pum- 
pelly are but phases of a large class of rocks; that between these typical 
kinds and the fine-grained, olivine-free diabase of the ordinary type there 
are various gradation-forms, in which the rounded grains of augite are 
mingled with more and more of the augite whose contours are determined 
by the feldspars; and also that, in the other direction, there are gradation- 
forms into aphanitic kinds, in which there is much non-polarizing, unindi- 
vidualized material. In extreme cases, as in a rock from Michipicoten, fig- 
ured at Plate IX., Figs. 1 and 2, the unindividualized base makes up the 
greater part of the rock. The presence of unindividualized base and the 
absence of recognizable olivine place these rocks among the diabase-por- 
phyrites, according to Rosenbusch’s nomenclature. But the ashbed- 
diabases are so plainly linked with these, both through intermediate kinds 
and through similarity of occurrence in the field, that all are considered 
here together. They all indicate rapidity of solidification, not only by the 
presence of unindividualized matter, but by the mode of occurrence of the 
augite. 
The groundmass of these rocks externally varies from light-gray to 
dark-gray in color, in more distinctly crystalline kinds; and is from light- 
gray to jet-black, and in one phase deep reddish-brown, in the less crystal- 
line kinds. With the former kinds the fracture is sub-conchoidal, with the 
latter very highly conchoidal and even glass-like, as in the case of some 
black semi-vitreous rocks that have a large development on Michipicoten 
Island. 
Under the microscope the tabular oligoclases usually make up most of 
the section in the more distinctly crystalline kinds, the more acid phases 
containing also orthoclase with the oligoclase. The augite is in irregularly- 
outlined particles, commonly very subordinate in quantity, though occa- 
sionally, as in some dense black rocks from Portage Bay Island, at the 
east end of the Minnesota coast, it makes up most of the section. The 
augite particles lie in the little spaces between the feldspars, which, how- 
ever, no individual particle ever completely fills. Several augite particles 
will occur together in such a space, or fill it along with the magnetite, or— 
and on the whole this is much more common—there is present more or less 
